1. Field of the Invention
The present invention relates in general to rotary actuators and in particular to a fine tracking rotary actuator in an optical disk drive system utilizing a magnetically biased bearing.
2. Description of the Prior Art
The typical optical data storage system reads and writes information to an optical disk. The optical disk is rotated at a relatively high rate of speed and a light source such as a laser beam is focused on one of a plurality of data tracks on the disk. The light is modulated or interrupted as a function of the recorded information and the modulated beam is utilized to reconstruct the recorded information. One important property of such a recording scheme is the ability to have a large number of tracks positioned closely adjacent to each other on the disk, resulting in the width of each track and the space between successive tracks being very narrow. The light spot must be maintained as a focused spot, and if the light spot is not maintained in alignment over the track being read or recorded upon, an error will be produced.
Multiple actuators which drive mirrors, lenses, and prisms are used to acquire fine positional control over the laser beam in directing it onto a data track. Mechanical positioning of the optical head carriage containing the primary mirror and lens actuators generally results in coarse track positioning in a regional section of the disk. Fine or individual track selection is accomplished through manipulation of optical element actuators. The fine tracking system includes means such as a mirror for changing the local angle of incidence of the laser beam along the optical path from the beam control unit which creates the beam to the focusing element in the carriage head in order to shift the focus spot laterally (i.e., radially). Fine rotary actuators allow precise control of the mirror in the radial direction relative to the data disk. Such radial control enables the system to accurately follow a data track on the disk or to switch to adjacent data tracks.
Because of extremely high track density and lineal recording densities in optical disk recorders, the slightest nonuniformity or play in the pivoting of the rotary actuator can introduce vibrational errors, misalignment errors, and dynamic errors. These errors become independent variables which have to be compensated for in aligning the laser beam as it reads the optical disk. These errors become significant in actuator alignment and operation at track spacings of about one micron and with a substantial linear density in which the cells are a micron or less in length along the track. Tracking errors occur from electronic errors in the track and seek servo controller controlling the optical elements directing the laser beam reading the disk. More significant are mechanical errors produced by wobbling and bouncing of optical elements. With the fine tracking rotary actuator loose toleranced bearings can create such wobbling and bouncing, resulting in large variations in the actuator's true angle and position and an imprecise alignment of the actuator's optical element. Standard shim and ball bearings which have significant play due to normal wear, product defects, and manufacturing variations are not well suited for the miniature and precise requirements of optical disk recording systems.
The same problems occur in hard disk recorders in which the magnetic transducer is rotatably mounted for track crossings and for track seekings. The high track densities of hard disks makes them susceptible to alignment and tracking errors created by loose tolerances in rotatable actuator bearings.
The prior art has made advances in creating higher tolerance bearings, but at the level of accuracy required for optical disk storage systems such bearings are costly to make, difficult to assemble, and easily malfunction. Accordingly, it has been desirable to provide a simple and accurate control of an optical disk rotary actuator at a high level of rotational tolerance.
One particular method of creating higher tolerance bearings for fine rotary actuators has been to create a magnetic bias force to firmly hold the rotating component of the rotary actuator against the actuator bearings. This magnetic bias force maintains a constant pressure against the bearings regardless of the rotational position of the actuator. Thus, the rotating component remains in a highly toleranced position with the bearings.
The prior art has created this magnetic bias force by inducing an electromagnetic field in the actuator. Such a device was created in Pat. No. 5,097,361. The rotating component has a magnet attached on each side. An electrical coil is placed in close proximity to each magnet. When current flows through each coil in the same direction, an electromagnetic force forces the magnets and rotating component in the same direction. The rotating component has a pivot point which is forced against a bearing surface by this electromagnetic force. This holds the rotating component in tight contact with the bearing surface.
In order to rotate the component, a differential in the coil currents is created. This changes the strength of the magnetic fields, produces unequal forces on the ends of the rotating component, and forces the component to rotate. In summary, large currents sent through both coils holds the component in high tolerance with the bearing surface. Rotation is achieved by slightly varying the magnitude of current through each coil.
Although this method achieves the high tolerance rotation desired, there are significant difficulties with the method. First, the high currents required to create the magnetic bias force consumes significant levels of power. It would be desirable to create a method which can create the advantages of a magnetic bias force without such a high consumption of power. Second, because of the high levels of current required to maintain the magnetic bias force, accurately controlling the variations in those currents in order to rotate the actuator is less precise. It would be desirable to create a method of magnetic bias force in which all the current going to the coils was used to rotate the actuator. This would allow low levels of current to be used which is much easier to precisely control. Moreover, during high frequency operation when the actuator is changing direction at very high speeds and rates of acceleration, the high currents degrade performance.